Many commercial products exist and numerous patents have been issued directed to the design and construction of impact attenuators or barriers to control the deceleration of an errant vehicle as it approaches an obstruction or hazard on, or adjacent to, a highway. Several prior art patents will be discussed below. This invention is concerned primarily with impact attenuators that are mounted on the rear of a construction vehicle, commonly called truck-mounted attenuators (TMA), although it is not limited thereto. The invention also has applicability to more permanent attenuator installations such as those of the type used around fixed highway structures especially where space is limited.
A review of some patents and commercial literature of TMAs illustrates a wide variety of designs which appear to have evolved by trial and error with little attempt to optimize the design to handle a wide variety of impacting vehicle kinetic energies. Thus, such existing devices generally have a fixed force versus deflection function that provides the same resisting force to the impacting vehicle regardless of that vehicle's mass or velocity.
The primary purpose of a TMA is to protect construction personnel from death or injury caused by a vehicle which mistakenly or accidentally intrudes into a construction zone. Secondarily, the TMA is designed to minimize the death and injury to the occupants of the errant vehicle. Ideally, the TMA should capture the impacting vehicle preventing it from being diverted either into adjacent traffic or off the road where it might impact a roadside structure such as a utility pole. Preferably, the TMA should even decelerate the vehicle at an acceptable level, such as 15 Gs, regardless of the mass or velocity of the impacting vehicle. Additionally, it is desirable for the TMA to be low cost, reusable after an impact, easily transported, light weight, easily shipped, easily stored, etc. No TMA on the market today satisfactorily meets all of these requirements. Therefore, there is a dire need for such a TMA that is the subject of this invention.
It is clear from the patents and commercial literature that many mechanisms exist for absorbing energy of an impacting vehicle into a TMA. These include a variety of structures that depend on the bending of metal or plastic, devices that utilize water, foam rubber, plastic etc. in a variety of energy absorption modes. Frequently, the energy dissipated by the system is part of the structure of the device. In fact, the prior art inventions have frequently confused the functions of structure and energy absorption. The instant invention therefore centers on the separation of these two functions of supporting structure and energy dissipation and optimizing these functions separately.
The basic problem to be solved by a TMA design is to capture an impacting vehicle and to decelerate it at an approximate constant value that is relatively independent of the velocity and mass of the impacting vehicle. It is also desirable for the impactor to be resetable and that it can be easily collapsed for transportation, shipping, storage etc. It is noteworthy that none of the TMAs on the market today are reusable and therefore invariably require replacement after an impact.